1. Field of the invention
The present invention is directed to light with short wavelengths (300-600 nm) for pulsed laser bonding of metal electrical members in microelectronic applications. In one embodiment a frequency doubled pulsed Nd:YAG laser beam at 1064 nm wavelength is directed to a nonlinear crystal to produce a laser beam with 533 nm and 1064 nm wavelengths.
2. Description of Related Art
Lasers used in microelectronic bonding provide concentrated localized heating. This is especially useful for soldering or welding bonds on electrical interconnects such as customizable copper/polyimide substrates with fine pitch dimensions, low thermal stress tolerance or heat sensitive components.
Yttrium-aluminum-garnet (YAG) crystal doped with neodymium (Nd) can produce a laser beam with a fundamental wavelength of about 1064 nm (1.064 microns) which is infrared radiation (IR) and invisible to the human eye. It has been found that Nd:YAG lasers provide a desirable balance between maximizing the absorption of metals and minimizing the absorption of polymer substrates. The use of 1064 nm Nd:YAG lasers for bonding electrical members in microelectronics is well known in the art; see, for instance, U.S. Pat. No. 4,697,061 to Spater et al, U.S. Pat. No. 4,845,355 to Andrews et al, and U.S. Ser. Nos. 07/405,377 and 07/558,127 to Spletter et al, each of which is hereby incorporated by reference.
Nd:YAG lasers at high frequencies in the range of 300-600 nm wavelengths, in particular the frequency doubled 533 nm wavelength (green light), can provide a substantial increase in the amount of laser energy absorbed by appropriate metal electrical members as they are heated and bonded. M. Greenstein in "Optical Absorption Aspects of Laser Soldering for High Density Interconnects," Applied Optics, Vol. 28, No. 21 (Nov. 1, 1989) points out that for both gold and copper metallurgies, the 533 nm wavelength provides significantly more absorption of laser energy than 1064 nm wavelength. Furthermore, as the temperature increases the predicted optical absorption of gold and copper increases as well. For example, Greenstein found that gold plated copper surfaces of TAB lead frames absorb about 40% energy from a pulsed Nd:YAG laser beam at 533 nm wavelength versus about 1%-5% energy absorption at 1064 nm wavelength.
533 nm wavelength radiation can be generated by directing a 1064 nm wavelength pulsed Nd:YAG laser at a nonlinear doubling crystal such as potassium titanyl phosphate. The crystal is affected by the electrical field of the laser beam to produce light with a frequency in the second harmonic at 533 nm wavelength. Upon applying 1064 nm wavelength radiation to the crystal typically 5%-10% is converted to 533 nm radiation and 5%-10% is lost as heat, reflections, etc. Unfortunately the crystal is highly sensitive to orientation and impinged power, and is prone to damage unless used in a very controlled environment. Water cooling is often provided to keep the crystal temperature approximately 30.degree.-40.degree. C.
While recently developed of GaAlAs diode lasers produce efficient and stable 533 nm wavelengths and look promising, current laser powers are in the tens-of milliwatts range which is not sufficient for microelectronic bonding.
Nd:YAG laser outputs can be continuous wave (CW), shuttered with an acousto-optic or electro-optic device (Q-switched), or pulsed. While these outputs are each at 1064 nm wavelength, the peak output powers differ widely and these differences can have a profound effect on the suitability of any Nd:YAG laser for a particular application.
Most currently available frequency doubled Nd:YAG lasers are either Q-switched or CW and as such are not suitable for microelectronic bonding. CW frequency doubled Nd:YAG lasers produce an energy flux that is difficult to control and tend to thermally shock and damage a bond site. Q-switched frequency doubled Nd:YAG lasers produce extremely high peak power for short pulse widths, e.g. kilowatts for nanoseconds, and as such drill or cut through the electrical members instead of bonding them by welding or solder reflow.
Frequency doubled pulsed (FDP) Nd:YAG lasers are typically used for range finding and night vision applications. Commercial vendors include Lumonics Corp. and Kigre Corp. The 533 nm wavelength creates green light which is well absorbed by the human eye. However the lasers employed for these applications have low output power and high repetition rate and therefore are not suitable for microelectronic bonding.